The NSA's wide-ranging surveillance programme should be curtailed, says hardware-maker Cisco in a letter to President Obama.

Cisco boss John Chambers said faith in US technology companies was being eroded by the NSA's activities.

The letter comes after whistleblowers revealed the NSA regularly intercepted Cisco hardware to help it gather information on potential targets.

Mr Chambers said the NSA should be held to higher "standards of conduct".

The first allegations about NSA staff intercepting deliveries of Cisco hardware came from papers given by whistleblower Edward Snowden to journalist Glenn Greenwald.

"If these allegations are true," wrote Mr Chambers in a letter published in the Financial Times, "these actions will undermine confidence in our industry and in the ability of technology companies to deliver products globally."

In addition, pictures have circulated online of NSA staff opening packing crates containing Cisco gear. The NSA is believed to have intercepted and altered the networking hardware so the agency can more easily gather information about the people and companies it targets.

"We simply cannot operate this way," Mr Chambers wrote. "Our customers trust us to be able to deliver to their doorsteps products that meet the highest standards of integrity and security."

To restore confidence in Cisco and other tech firms, President Obama should draw up new rules to govern the way the NSA operates to ensure its conduct was held to a high standard, he said.

The allegations about Cisco were not the last to be published about NSA surveillance, said Glenn Greenwald in a wide-ranging interview with GQ. Mr Greenwald said he was saving some of the biggest leaks until last.

This final tranche of information would detail who in the US the NSA has been targeting and why it chose those targets, said Mr Greenwald.

"Are they political critics and dissidents and activists? Are they genuinely people we'd regard as terrorists?" he said to the magazine. "What are the metrics and calculations that go into choosing those targets and what is done with the surveillance that is conducted?"

Freedom banner

The continuing revelations about the NSA have also prompted action by China which has said it plans to beef up its internet security defences in response to "overseas hostile forces".

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Media captionJournalist Glenn Greenwald tells the BBC how he almost missed out on the Snowden scoop.

Such forces were using the net to "penetrate and destroy" China, said Wang Xiujun, the state official in China's Internet Information Office who directs the nation's net regulation system.

"A few countries have used their superiority in internet resources and information technology to conduct large-scale internet surveillance and to steal a large volume of political, economic, military and corporate secrets", Mr Wang is reported to have said.

Information from documents released by Edward Snowden suggest the NSA had managed to get at servers run by Chinese communications giant Huawei to look at documents and monitor messages passing between executives.

In addition, he said, many people were using the banner of "internet freedom" to attack and slander China and undermine its stability.

The improved defences against external threats and improved monitoring and censorship of internal activity would help China win "the struggle for ideological penetration", Mr Wang said.